The present invention relates generally to a filter arrangement for cleaning particulate laden process gas streams such as provided by coal gasification processes or the incineration of organic material. More particularly, the present invention is directed to a filter provided in combination with a fluidized bed of granular material and formed in-situ by depositing a gas permeable and substantially particulate-impermeable layer of granular bed material on an exposed surface of a porous membrane.
Gas filtering mechanisms for efficiently and effectively removing gas borne particulate material and gaseous pollutants such as sulfur oxides and hydrogen sulfide from process gas streams are becoming increasingly important in the development and use of coal utilization devices such as combined cycle turbines, direct fired combustion turbines, coal gasification systems, and fuel cells. The successful operations of such coal utilization devices require reliable and efficient gas stream clean-up mechanisms which can remove particulate and gaseous pollutants from the process gas stream before the stream is discharged into the environment or employed in a process-gas utilization device such as gas turbine or fuel cell. Also, the incineration of waste material such as low level radioactive waste containing organic material such as clothing, liquids including oily waste, and building materials is of present interest in order to reduce the increasingly large inventory of such radioactive waste. The filtering of the gas stream emanating from such an incinerator is required to prevent the discharge of hazardous radioactive particulate material into the environment.
Presently available filtering mechanisms for cleaning high temperature and high pressure gas streams laden with particulate material and gaseous pollutants such as those discharged from fluidized beds and gasification systems have been largely successful in removing about 99% or more of the particulate material from such gas streams. These filtering mechanisms include screenless granular bed filters, ceramic cross-flow filters such as described in U.S. Pat. No. 4,343,631, and metal or ceramic candle filters as described in U.S. Pat. No. 2,548,875.
Screenless granular bed filters utilize the granules in a fixed or moving bed as the particulate capturing mechanism to remove greater than about 99% of the particulate material from relatively slow moving particulate-laden gas streams passing through the bed at substantially constant gas pressures and velocities. However, this type of filter has some problems with respect to variations in gas pressure and gas velocity since it is relatively sensitive to any changes in the pressure of the gas and to gas streams contacting the bed at relatively high velocities in that such changes in pressure and gas streams passing through the bed at relatively high velocities tend to inhibit the capture of a desired percentage of the gas-borne particulates from the particulate laden gas streams.
The cross-flow filters and the candle filters employ porous ceramic or metal filtering elements which are permeable to gas and essentially impermeable to particulate material including even the very fine particulates in the 0+ to 5 micron range so as to effectively remove 99+% of the particulates from the process gas stream passing through the filtering elements. While these filters have been found to be efficient for removing virtually all of the particulate material from process gas streams, these filters do encounter a problem in that the particulate material borne by the process gas forms a relatively dense particulate deposit or filter cake on the surface of the filter. As this build-up of the filter cake continues, the flow of gas through the filter becomes increasingly restricted to cause an excessive pressure drop across the filtering element so as to necessitate relatively frequent removal of the filter cake, such as by high pressure blow-back procedures.